The present invention relates to a heat-resistant cast steel excellent in oxidation resistance, thermal crack resistance, heat deformation resistance, etc. as well as castability and workability, and a process of producing such a heat-resistant cast steel, and parts such as combustion chambers and exhaust equipment members for internal-combustion engines which are made of such heat-resistant cast steel.
Generally, materials composing parts for exhaust equipment members and combustion chambers of gasoline engines and diesel engines of automobiles are empirically selected, by considering the temperature of exhaust gas at a full-load operation of engines, the total exhaust gas energy determined by the temperature of exhaust gas and the amount of exhaust gas emitted per hour, the shapes of parts, constraint conditions for parts, and heat capacities of parts for exhaust gas-cleaning members which determine the time to reach the activation temperature of exhaust gas-cleaning catalytic converters from the cold-start of engines, etc.
Since exhaust equipment members for automobiles, for instance, prechambers, port liners, exhaust manifolds, turbocharger housings, exhaust outlets connected right under turbochargers, and parts for exhaust gas-cleaning members such as exhaust gas-cleaning catalytic converters, etc. are likely to be oxidized or subjected to thermal stress when operated at an extremely high temperature, materials having relatively good heat resistance, such as high-Si spheroidal graphite cast iron, austenite spheroidal graphite cast iron containing a large amount of Ni, and in a few cases a heat-resistant austenite cast steel SCH12 have been employed conventionally.
Particularly in case where the temperature of exhaust gas at a full-load operation is 900.degree. C. or lower, high-Si spheroidal graphite cast iron and FCD400 (JIS Standard) cast iron, etc. are mainly employed for exhaust manifolds for engines of an uncontrolled air intake-type, exhaust gas-cleaning catalytic converter containers connected to the outlets of the exhaust manifolds, etc. Also, high-Si spheroidal graphite cast iron and austenite spheroidal graphite cast iron are employed for exhaust manifolds for supercharger-equipped engines and turbocharger housings, etc. in view of functional requirements for these parts. In the latter case, high-Si spheroidal graphite cast iron, FCD400 (JIS Standard) cast iron, etc. are mainly employed for exhaust gas-cleaning catalytic converter containers connected to the outlets of the turbocharger housings.
On the other hand, in the case of super high-performance engines with which the temperature of exhaust gas at a full-load operation exceeds 900.degree. C., austenite spheroidal graphite cast iron and a high-alloy, heat-resistant, ferritic cast steel are employed for exhaust manifolds for supercharger-equipped engines, and in some cases austenite spheroidal graphite cast iron is also employed for exhaust manifolds for high-performance engines of an uncontrolled air intake-type. Also, a high-alloy, heat-resistant, ferritic or austenite cast steel has become adopted for turbocharger housings of such super high-performance engines.
However, because of the recent strict regulations of the emission of exhaust gas, further improvement of the efficiency of the purification of exhaust gas at the cold-start of engines has been required. To fulfill this objective, it is necessary to reduce the heat capacity of each member from an exhaust manifold to an exhaust gas-cleaning catalytic converter equipment, so that the temperature of the catalytic converter can reach its activation point as soon as possible after the cold-start of an engine. Also, in order to improve the fuel efficiency and to decrease the amount of CO.sub.2 emitted, it is necessary to make parts of automobiles including engine parts extremely light and to improve the energy efficiency by high-temperature combustion.
For this purpose, exhaust parts constituted by thin and light welded pipes have lately been produced by pressing or bending rolled sheets or pipes made of ferritic stainless steel such as SUS410, SUS430, etc. and afterwords by welding them, and such exhaust parts have become popular. However, since such parts having welded structures, for example, pipe-gathering portions of exhaust manifolds, have complicated structures, their production costs are so high. In addition, since such parts are subjected to great thermal stress in many cases, it is difficult to obtain the parts having good durability (such as heat deformation resistance, thermal crack resistance, etc.).
Therefore, in order to solve such a problem on parts which are difficult to form and weld, exhaust equipment members consisting of cast parts having complicated shapes and made of a so-called high-alloy, heat-resistant cast steel described above and bent pipes welded to the cast parts are employed in some cases.
For example, in the case of an engine of an uncontrolled air intake-type, as is shown in FIG. 6, a smaller exhaust gas-cleaning catalytic equipment (a secondary catalytic converter) 4 effective for a cold-start is fitted directly to the exhaust manifold 1, and a bigger exhaust gas-cleaning catalytic equipment (a primary catalytic converter) 7 is disposed on the downstream side of the smaller catalytic equipment 4. The secondary catalytic converter container 4 is welded to the downstream end of the exhaust manifold 1, and the primary catalytic converter container 7 is welded to a front tube 6 which in turn is welded to the downstream end of the secondary catalytic converter 4. Because of such a layout, a thermal capacity (thermal inertia) of the whole exhaust equipment member decreases, and heat is hardly taken away from the exhaust gas on its way. Therefore, a capacity for the purification of exhaust gas by a catalyst at a cold-start increases remarkably.
Since the exhaust gas at a high temperature exceeding 900.degree. C. passes through these parts of the exhaust equipment at a full-load operation of an engine, it is strongly desired that the parts of the exhaust equipment should have excellent heat resistance (oxidation resistance, thermal crack resistance, and heat deformation resistance).
In the background described above, U.S. Pat. No. 4,790,977 discloses as an alloyed steel which has excellent oxidation resistance and creep strength at a high temperature, an alloyed steel consisting of a ferrite phase and having a composition consisting essentially, by weight, of:
C: about 0.01-0.3%, PA1 Mn: about 2% or less, PA1 Si: over 2.35% and up to about 4%, PA1 Cr: about 3-7%, PA1 Ni: about 1% or less, PA1 N: about 0.15% or less, PA1 Al: less than 0.3%, PA1 at least one element for forming carbide and nitride (Nb, Ta, V, Ti, Zr): 1.0% or less, PA1 Mo: up to 2%, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 W and/or Co: 0.1-2%, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 Rare earth element and/or Y: 0.1% or less, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 Mg and/or Ca: 0.005-0.03%, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 B: 0.001-0.01%, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 W and/or Co: 0.1-2%, PA1 Rare earth element and/or Y: 0.1% or less, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 W and/or Co: 0.1-2%, PA1 Mg and/or Ca: 0.005-0.03%, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 W and/or Co: 0.1-2%, PA1 B: 0.001-0.01%, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 W and/or Co: 0.1-2%, PA1 Rare earth element and/or Y: 0.1% or less, PA1 Mg and/or Ca: 0.005-0.03%, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 W and/or Co: 0.1-2%, PA1 Rare earth element and/or Y: 0.1% or less, PA1 B: 0.001-0.01%, and PA1 Fe and inevitable impurities: balance. PA1 C: 0.05-0.25%, PA1 Si: 2.5-3.5%, PA1 Mn: 2% or less, PA1 Cr: 4-8%, PA1 N: 0.05% or less, PA1 W and/or Co: 0.1-2%, PA1 Rare earth element and/or Y: 0.1% or less, PA1 Mg and/or Ca: 0.005-0.03%, PA1 B: 0.001-0.01%, and PA1 Fe and inevitable impurities: balance.
However, it turns out that such an alloyed steel consisting of a ferrite phase does not always exhibit sufficient heat resistance, particularly heat deformation resistance when exposed to a high temperature of 800.degree. C. or higher.